1. Field of the Invention
This invention relates to the method of forming a metal semiconductor field effect transistor and the product formed thereby. More particularly, this invention relates to the method of forming such a field effect transistor having a self-aligned gate.
2. Description of the Prior Art
A metal semiconductor field effect transistor, MESFET, differs from a traditional device such as a junction field effect transistor, JFET, and the metal oxide semiconductor field effect transistor, MOSFET. With a standard MOSFET, an oxide insulative layer separates the transistor gate from the semiconductor substrate between the source and drain of the transistor. In the JFET, the gate is in contact with the substrate, which, however, has nevertheless been appropriately doped so that a PN junction is formed. In a MESFET, the gate makes direct contact with the silicon substrate so as to form a Schottky diode by a metal-silicon interface. In the JFET, the PN junction is a minority carrier device while in the MESFET, the Schottky diode is a majority carrier device. In the MESFET and JFET devices, the current conduction is within the bulk of the silicon substrate. While in a MOSFET, the current conduction is along the oxide-silicon interface.
With the conventional methods of the prior art for forming a MESFET, the source and drain areas are first created by diffusion or implant, the channel region is then implanted, and an oxide layer deposited thereover. A via is then opened through the oxide to the silicon substrate between the source and drain areas. The metal gate is then formed in the via which is of a sufficiently small dimension to make certain that the gate does not make ohmic contact with either the drain or source areas as such contact would short out the gate. The alignment of the gate, then, is critical which in turn requires that more space be allowed for the gate. This in turn diminishes the packing density of the circuit. A particular example of this process is disclosed in the Buiatti, U.S. Pat. No. 4,048,712.
It is then an object of the present invention to provide an improved method of forming a metal silicon field effect transistor.
It is another object of the present invention to provide a metal silicon field effect transistor having reduced space requirements.
It is still another object of the present invention to provide an improved metal silicon field effect transistor having a self-aligned gate.